"Pairing" Issue connecting Wireless Keyboard via BlueTooth. How do i find passcode?

I recently bought a new MacBook Pro. I am attempting to connect it to my Apple Wireless Keyboard (that was connected to my previous MacBook) via BlueTooth (it is discoverable but i dont know passcode) and it keeps saying it is "attempting to pair" it and "the pairing was unsuccessful." How do i find passcode? I no longer have the box it came in, my keyboard is several years old. I have tried using no passcode or connecting automatically or using 0000 and none of those work, I keep getting the same error. I also have a mouse that connects fine via Bluetooth, it is just the keyboard that is not pairing. Any ideas to save a trip to the Apple Store? Thanks!